avoid the Canon 90-300 (both USM and w/out USM) and 100-300 as they're slow AF and optically fairly poor.
the 70-300 IS is a very nice lens. sure, it's not an 'L', but the contrast is decent, there isn't too much chromatic aberation, and the IS works well (for animals, not sports). the 75-300 was a decent lens, but the IS was the first - very hard on batteries and not as good as the recent generations.
and yes, the Canon 70-200 models are all very good.
sigma does make a few very good zooms:
-120-300 F2.8 EX - fantastic lens and worth the price
-50-500 EX (aka 'Bigma'). this is my primary outdoor sports lens. i personally prefer this over the Canon 100-400 IS because i hate push/pull
-70-200 F2.8 EX - great colour, quick AF, and fantastic sharpness. almost as good as the Canon 2.8L
and a few decent Sigma zooms
-100-300 F4 EX colour is pretty good - beautiful bokeh for a lens this long. not as sharp until about F7.1
-300-800 EX - only lens at this distance. needs a very good tripod. not as sharp as most EX glass. if you have to ask the price, it's not for you...
also the 300 F2.8 is a great lens. sharp almost wide open, fast (AF and aperture), and great contrast. the only downside is for all that money, you want want a white label (aka look at me with my 'L'!).
avoid the following Sigma lenses:
-170-500 - soft and slow
-135-400 - slow AF. not too bad over F8. needs to be rechipped if you are getting the non DG version. not worth the price, IMHO.
-28-200, 28-300 - who are they kidding with the red stripes? these are often sold as quantary through Wolf/Ritz. slow, no Hypersonic. lots of chromatic issues.
-80-400 EX OS - the OS is no where as nice as Canon IS. it under-compensates, over-compensates, and eats the batteries. mode 2 when shooting landscape orientation isn't as bad - presumably because it's only compensating in one direction (designed for panning like motorsports). with OS off, it's nicely sharp and beautiful bokeh. but it's only a few bucks cheaper than the 100-400 IS L...